Furfural is produced on an industrial scale from agricultural wastes, for example, oat hulls, bagasse, corn cob residue and saw dust and like wood residues or straw. All of these materials contain pentosan which is initially hydrolyzed to pentose with the take-up of water in accordance with the relationship: EQU (C.sub.5 H.sub.8 O.sub.4).sub.n +n H.sub.2 O.fwdarw.n C.sub.5 H.sub.10 O.sub.5 ( 1)
The pentose is then converted to furfural in a dehydration reaction in accordance with the relation: EQU C.sub.5 H.sub.10 O.sub.5 -3 H.sub.2 O.fwdarw.C.sub.5 H.sub.4 O.sub.2 ( 2)
The process can be carried out either on a batch basis or continuously.
In the batch process used generally throughout the world, utilizing the principles of the Quaker Oats process described by H. J. Brownlee and Carl S. Miner in Industrial Development of Furfural, Ind. Engng. Chem. 40 (1948) 201-204, comminuted raw material moistened with dilute sulfuric acid is treated at 153.degree. C. and 5 ATM with steam in a reactor which rotates slowly to circulate the raw material. Steam supply and product vapor discharge are effected via the stub shafts of the reactor. The process conditions for the reactor are optimized and then maintained for industrial scale production. The residence time of a charge under these process conditions is usually about 5 hours.
The much later developed continuous process of Escher Wyss or Rosenlew utilizes the Quaker Oats principle of moistening the raw material particles with dilute sulfuric acid, but utilizes tall-shaft reactors in which the raw material is introduced from above through a gate in the reactor. The raw material descends in counterflow from the steam which is introduced from below in a fluid-bed system and the vapor is discharged. The reactor operates at a temperature of up to 184.degree. C. and at a pressure of up to 11 ATM. The residence time in the reactor is about 1/2 hour.
Problems with this continuous process result from the gating of the solids into and out of the reactor, and from the fact that the steam must not only serve as a reactant but also must function as a carrier medium which reduces the efficiency because of the coupling of the chemical process with the mechanical process, etc.
Because of the fact that the particle size of the raw material is not uniform, the residence time range in the reactor for individual particles is quite wide which gives rise to a number of drawbacks effecting the chemical process.
Apart from the aforementioned processes used commercially for the production of furfural, mention can be made of a number of known processes in which furfural is recovered as a practically unavoidable by-product.
This is case, for example, in the cleaning of waste waters from the woodworking industry or the manufacture of ethanol.
In a process for the production of glucose, an intermediate product in ethanol production, as described in the report "High Temperature Acid Hydrolysis of Biomass Using an Engineering-Scale Plug Flow Reactor: Results of Low Solids Testing" of Brennan, Hoaglund and Schell, (Bioltechnology and Bioengineering Symp. No. 17; 1986), industrial tests are shown to yield furfural in reduced quantities as a by-product and insofar as possible as a product which is utilized to improve the economies of the process by increasing the cash flow of the ethanol-producing apparatus.
In this process comminuted wood together with dilute sulfuric acid is heated under pressure with saturated steam in a plug-flow reactor.
In the reactor cellulose is hydrolyzed to glucose, a product which is then fermented for the production of ethanol. Water is separated by expansion into an expansion vessel in the form of steam. The steam or water vapor contains small amounts of furfural at low concentrations which appear to result as a by-product from the hydrolysis of the wood to glucose.
U.S. Pat. No. 4,533,743 describes a furfural-making process in which furfural is made from a pentose solution.
The production of the pentose solution from a pentosan-containing solid is not described in this patent. In this process, the solids-free pentose solution after traversing the reactor is cooled by heat abstraction without the formation of a vapor phase. The yield of furfural is comparatively small in this system as is the furfural concentration in the products obtained.
Both the batch process and the continuous process as well as the experimental production of ethanol with a plug flow reactor as described, operate with acid medium, usually with sulfuric acid, because the speed of the reactions of relations (1) and (2) are directly proportional to the hydrogen ion concentration. Acceleration of the reaction represented by the equation (2) is highly desirable because this reaction is the slowest step and therefore is the rate-determining step of the overall reaction. From equations (1) and (2), it is possible to ascertain that the overall reaction for the production of furfural from pentosan-containing raw material is: EQU (C.sub.5 H.sub.8 O.sub.4).sub.n .fwdarw.n C.sub.5 H.sub.4 O.sub.2 +2n H.sub.2 O (3).
From this reaction equation it can be readily seen that the maximum possible furfural yield is 72.7% of the pentosan. In practice, however, the best that can be obtained is 1/3 of this value. Even raw materials which have relatively high pentosan contents and thus the greatest potential for the production of furfural, like oat hulls and corn cob residues, contain about 32% pentosan so that the practical achievable furfural yield can only be a maximum of about 10% of the dry substance of the raw material which is processed.
Since the raw materials have bulk densities or bulk specific gravities of about 250 kg/m.sup.3, with usual reactor fillings of about 50% and required residence times up to 5 hours, conventional processes require large reactor volumes.
Not only do large reactors take up large amounts of valuable space, but because of the fact that the reactors must be pressure type and corrosion resistant, they also involve high capital cost. Another problem with earlier methods from which furfural may be recovered is dealing with the residues which are contaminated with sulfuric acid.